In recent years, the spread of electrostatic copiers or laser beam printers employing electrophotographic technologies has been remarkable. In harmony with this trend, the properties of printing graphics or photographs with high reproducibility have been desired. With the concurrent progress of techniques for the meticulous creation of latent images, there has been a demand for the accurate printing of thin lines by development. To fulfill these requirements, the particle sizes of magnetic toners have been made smaller. Magnetite added as a magnetic black pigment should also be formed into fine particles.
Magnetite in current use has a particle size of 0.1 to 0.3 .mu.m. Thus, the need for ultrafine magnetite with a particle size of 0.1 .mu.m or less is growing. However, 0.1 .mu.m or less ultrafine magnetite particles with a black tint sufficient for use as a black pigment is nonexistent as yet.
Fine magnetite particles for use as a magnetic black pigment to be incorporated into a magnetic toner are generally those obtained by adding an alkali to ferrous sulfate to obtain a suspension of Fe(OH).sub.2, and air oxidizing the suspension at a suitable temperature. Various attempts to obtain fine particles 0.1 .mu.m or less in diameter by this method have been made based on studies of conditions, such as pH, rate of oxidation and reaction temperature. However, it is difficult to make the particle size smaller than 0.1 .mu.m while retaining a black color.
It is well known to add an alkali to a mixed aqueous solution of a ferrous salt and a ferric salt to cause coprecipitation, and age the coprecipitates at 50 to 100.degree. C., thereby forming ultrafine magnetite with a particle size of 0.01 .mu.m or less.
The so obtained ultrafine particles are so fine that when they are taken out into the air, they are oxidized, making their use for a magnetic toner or the like difficult. Their use is restricted to applications excluded from the air, such as magnetic fluid.
As a method for overcoming the drawbacks of the foregoing two representative methods, Japanese Patent Publication No. 33655/90 discloses a process for producing ultrafine magnetite with a particle size of about 0.01 to 0.025 .mu.m.
The technique disclosed there makes the content of a ferrous salt higher than in the composition of magnetite when obtaining ultrafine magnetite by coprecipitation, to cause ferrous hydroxide to remain after coprecipitation, and air oxidizes the ferrous hydroxide to grow air oxidation-induced magnetite on the coprecipitated ultrafine magnetite particles, thereby somewhat enlarging the ultrafine magnetite particles for stabilization.
Even this method, however, poses the problem that when the particles are taken out into the air, they become reddish and cannot serve as a magnetic black pigment.
The present situation is, therefore, that there is no ultrafine magnetite 0.1 .mu.m or less in particle size which is black enough to be used as a magentic black pigment.
The object of the present invention is to provide black ultrafine magnetite particles for a black magnetic toner that earlier technologies were unable to produce, and a process for producing the black ultrafine magnetite particles.